(1) Field of the Invention
This invention relates to an electroconductive paste, and particularly to an electroconductive molybdenum paste suitable for filling throughholes.
(2) Description of the Prior Art
Heretofore, multi-layered, ceramic circuit boards have been prepared by providing througholes on a green sheet made from ceramic powders and an organic binder, printing on the green sheet an electroconductive paste, thereby forming an unfired circuit pattern thereon, filling the throughholes with the electroconductive paste, laying a desired number of the green sheets one upon another, pressing the sheets by a hot press, etc. and sintering the pressed sheets at 1,500.degree. to 1,600.degree. C.
The higher the circuit density and the LSI load density of the thus prepared ceramic circuit board, the larger the cracks developed near the throughholes.
To prevent the development of cracks, the following steps (1) to (5) have been proposed:
(1) To add Al.sub.2 O.sub.3 to the paste to make the thermal expansion coefficient approach that of the ceramic.
(2) To add a low melting glass to the paste to lower the solidification temperature and reduce the residual stress.
(3) To increase the particle size of metal powders for use in the electroconductive paste to prevent sintering and disperse the thermal shock stress.
(4) To increase the mechanical strength of the ceramic.
(5) To use a plastically deformable metal as a conductor.
The electroconductive molybdenum pastes having the following compositions 1 and 2 are based on combinations of steps (1) and (5), and steps (1), (2) and (5), respectively.
Composition 1 (disclosed in Japanese Patent Publication No. 57-8799):
______________________________________ Mo powders (average particle 100 g size: 0.6 .mu.m) Mn powders 0-6 g 80 parts Al.sub.2 O.sub.3 powders 0-6 g by weight n-butyl carbitol 10 parts by weight ethyl cellulose 10 parts by weight ______________________________________
Composition 2 (disclosed in Japanese Patent Application Kokai No. 51-80965):
______________________________________ Mo powders (average particle 70 g size: 1-3 .mu.m) Glass frits (42 wt. % Al.sub.2 O.sub.3, 30 g 67.5 parts 54 wt. % SiO.sub.2, 2 wt. % by weight CaO, 2 wt. % MgO) Butyl carbitol acetate 7.5 parts by weight Ethyl cellulose 20 parts by weight Sarcosine 5 parts by weight ______________________________________
By filling throughholes with said pastes, development of cracks near the throughholes can be prevented even if the circuit density and LSI load density are increased.
However, by filling the throughholes on the green sheet with said pastes, the solvent in the pastes is absorbed into the green sheets and the pastes undergo volumic shrinkage. The shrinkage is also occasioned by drying and firing. Total shrinkage amounts to 30-40%. Thus, voids can be formed in the filled electroconductive pastes or hollows can be formed on the surfaces of electroconductive pastes filled in the throughholes [surface roughness: 6-20 .mu.m (R.sub.max)]. Thus, (a) resistance becomes higher in the throughholes and at the contact surfaces of multi-layers, and (b) a plating film is hardly formed on the hollows on the electroconductive paste in the throughholes at the surface of a multi-layered board. Furthermore, when the paste is filled by printing, the Mo powders and the binder in the paste are separated from each other by the squeezing pressure, so that the Mo powders are coagulated to form deep clearances extending from the paste surface. The clearances remain in the surface layer even after sintering [surface roughness: 6-20 .mu.m (R.sub.max)], and when a plating layer is formed on such surface layer, the plating pretreatment solution and the plating solution are retained in the clearances under the plating layer. This leads to corrosion at the interface between the Mo conductor and the plating layer, and the resistance may be increased or the electrocontact may be deteriorated while actually used.